Column select circuit

ABSTRACT

A column select circuit for a memory device is disclosed which, for unselected data lines, provides a high impedance output. Each data line, and a corresponding decoded address signal, is received by a gate which passes the logic state of the data line (inverted), if selected, to a driver. The decoded address signal is also communicated to the driver, for tri-stating the driver for unselected data lines. The driver consists of a p-channel pull-up and an n-channel pull-down, with an n-channel isolation transistor connected in series therebetween. The driver output is at the junction of the pull-up and isolation devices. The gates of the pull-up and pull-down transistors are connected to the output of the gate, with the gate providing a high logic level when not selected, turning off the pull-up device. The isolation transistor is also turned off when the data line is not selected, isolating the output from both the power supply node and ground, which allows the output to be pulled by photoconduction toward mid-rail in a transient radiation event. A plurality of gate/driver pairs are provided within a stage of the select circuit, each having their outputs connected together. The select circuit may be made up of a plurality of stages constructed as described above.

This invention is in the field of integrated circuits, and specifically in the field of decoder circuits for integrated memory circuits.

This invention was made with Government support under contract No. DNA 001-86-C-0090 awarded by the Defense Nuclear Agency. The Government has certain rights in this invention.

This is a division of application Ser. No. 07/130,769, filed Dec. 9, 1987, now U.S. Pat. No. 4,916,336.

Background of the Invention

Memory cell arrays in conventional random access memory circuits (RAMs) are generally organized into independently selectable rows and columns. Selected address signals presented to the address terminals of the device are decoded by a row decoder to select a row of the memory cell array to be communicated to sense amplifiers. Others of the address signals are decoded by a column decoder in order to select one or more of the bits in the selected row from or to which data is to be read or written. Such column decoders in conventional RAMs thus in effect perform a multiplexing operation to select one of the many possible bits in a selected row.

Referring to FIG. 1, a common column decoding and demultiplexing scheme is illustrated, realized for the example of a one-of-four selection. Signals A0 and A0 are the logical complement of one another, and constitute the least significant bit of the two address signals; similarly signals Al and Al present the true and false state of the second least significant address bit. AND gates 10₀ through 10₃ are connected to the various combinations of the four true and false address lines, to generate a high logic level at its output responsive to the proper combination of address signals at its inputs. For example, the output of AND gate 10₂ will be high responsive to address lines A0 and Al being high (address 10₂) A high output from an AND gate 10 will turn on the associated pass transistor 12₀ through 12₃, which will connect the corresponding data line D₀ through D₃ to line OUT. Lines D₀ through D₃ are the data lines driven by, for example, four sense amplifiers in a RAM device, among which the circuitry of FIG. 1 is to select for application to line OUT. Alternatively, lines D₀ through D₃ may be the actual bit lines of the memory device, prior to sensing, for such memories which utilize a single sense amplifier for multiple columns. As shown in FIG. 1, transistor 14 is connected between line OUT and the V_(dd) power supply, to precharge line OUT to V_(dd) when signal PC at the gate of transistor 14 is high. Line OUT is then either pulled down if the state of the selected data line D₀ through D₃ is low, or left high if the selected data line is high. As is well known in the art, data lines D₀ through D₃ can directly drive (i.e., discharge or keep) line OUT, or may in turn gate a discharge capacitor to discharge line OUT in the event of a logic low thereupon. Many other realizations of the column decoder shown in FIG. 1 may of course be realized using the pass gate concept, including domino logic realizations and other precharge-discharge schemes. In addition, many more pass gates may be used to decode more address lines to select a bit or bits from a group of data lines much larger (e.g., a group of 128) than the four illustrated in FIG. 1.

It is well known that transient gamma radiation exposure of a device containing a circuit such as is shown in FIG. 1 can cause the pass transistors 12, and also the precharge transistor 14, to conduct due to photoconduction, even though the transistors are nominally off. If these transistors conduct in the off state, the state of line OUT could become upset from that of the logic state of the selected data line D₀ through D₃, due to the V_(dd) power supply driving line OUT toward a high voltage through photoconducting transistor 14, and due to the unselected ones of data lines D₀ through D₃ pulling line OUT through their associated photoconducting pass transistors 12 to their logic level. If the sum of the photoconduction current through transistor 14 and the unselected pass transistors 12 reaches the level of the current through the selected one of pass transistors 12₀ through 12₃, an error can be produced. The worst case of such an event for the circuit of FIG. 1 is if the selected data line, for example line D₀, is at a low logic level and the unselected data lines D₁ through D₃ were at a high logic level. If the sum of the photoconduction current through the four "off" transistors 14 and 12₁ through 12₃ reaches the level of the current through the selected pass transistor 12₀, the logic low level presented by data line D₀ would not be detected.

It is therefore an object of this invention to provide a column decoder circuitry which is more tolerant of pass transistors conducting in the off state, such as photoconduction occurring in transient radiation events.

It is yet another object of this invention to provide such a column decoder which is constructed in stages, so that a weakened output level from an earlier stage can be boosted by a subsequent stage, further increasing the transient radiation tolerance of the device.

It is yet another object of this invention to provide such a column decoder which has low fan-in and fan-out stages, thereby providing reduced switching-time-to-temperature sensitivity.

Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification in conjunction with the drawings.

Brief Description of the Drawings

FIG. 1 is an electrical diagram, in schematic form, of a column decoder according to the prior art, utilizing a pass gate scheme.

FIG. 2 is an electrical diagram, in schematic form, of a stage within a column decode and select circuit constructed according to the instant invention.

FIG. 3 is an electrical diagram, in block form, of a three-stage column decode and select circuit constructed according to the invention.

Summary of the Invention

The invention may be incorporated into a multi-stage column decoder, where each stage receives a number of data lines from the prior stage, or from the memory array for the first stage, and a number of decoded address signals. Within each stage, logic is provided to gate each of the data lines responsive to its associated decoded address line. If the decoded address line is asserted for a data line, the state of the data line drives a tristate driver; the address lines which are not asserted cause the tristate driver to go to a high impedance state. The high impedance state for unselected drivers isolates their data lines from the output of the decoder stage, so that photoconduction caused by transient radiation pulls the output node toward mid-rail, rather than high or low, increasing the radiation tolerance of the circuit. The multi-stage design boosts the output of each stage, so that a logic level from a stage which is degraded by the transient radiation event is restored.

Detailed Description of the Preferred Embodiment

Referring now to FIG. 2, a stage 20 of a column decoder according to the invention will be described. For purposes of explanation, stage 20 in FIG. 2 will be described as the stage nearest the memory array; as will be further described below, the same stage design is equally applicable to subsequent stages within a multiple stage column decoder.

Stage 20 receives signals on data lines D₀ through D₃ as data signals, similarly as the decoder illustrated in FIG. 1. Stage 20 receives the address inputs COL0 through COL3 for the selection of one of the data lines D₀ through D₃ at terminal OUT₂₀. The address signals on lines COL0 through are decoded address signals, such that one of the lines COL0 through COL3 will be asserted (i.e., at a high logic level) to select one of the four data lines D₀ through D₃, while the other three of the four address lines COL0 through COL3 will not be asserted (i.e., at a low logic level). Accordingly, lines COL0 through COL3 of FIG. 2 correspond to the four outputs of AND gates 10₀ through 10₃ illustrated in FIG. 1.

Stage 20 may be considered to consist of four substages 20₀ through 20₃, each of which receives one of data lines D0 through D3, and the corresponding one of address lines COL0 through COL3. Referring to substage 20₀, NAND gate 22₀ receives data line D₀ at one input, and its corresponding address line COL0 at its other input. The output of NAND gate 22₀ is coupled to the gate of p-channel transistor 24₀, and to the gate of n-channel transistor 28₀. Address line COL0 is connected directly to the gate of n-channel transistor 26₀. Transistors 24₀, 26₀, and 28₀ have their source-to-drain paths connected in series between power supply node V_(dd) and ground, with p-channel transistor 24₀ serving as the pull-up transistor in the push-pull driver formed by the three transistors 24₀, 26₀, and 28₀, n-channel transistor 28₀ serving as the pull-down transistor, and transistor 26₀ serving as an isolation transistor. The output of substage 20₀ is at the node between the drains of transistors 24₀ and 26₀, and this node is directly connected to output line OUT₂₀, which is the output of stage 20. In operation, in the event that address line COL0 is asserted to select data line D₀, the output of NAND gate 22₀ will be the logical complement of the logic state of data line D₀. With line COL0 at a high logic level, transistor 26₀ will be turned on, so that transistors 24₀ and 28₀ will serve as an inverting push-pull driver responsive to the logic state of the output of NAND gate 22₀. Accordingly, when selected, the logic state of data line D₀ (twice inverted) will appear at the node between the drains of transistors 24₀ and 26₀, and communicated to the output of stage 20 at line OUT₂₀.

For unselected data lines D₀ through D₃ the corresponding address line COL0 through COL3 will be at a low logic level. Referring to substage 20₁ as an example, line COLl will be low when address line COL0 is at a high logic level. The output of NAND gate 22₁ will thus be forced to a high logic level regardless of the state of data line D₁, turning off transistor 24₁. In addition, the low logic level of line COLl will turn off transistor 26₁, connected between the drains of transistors 24₁ and 28₁. For the example where data line D₀ is selected, line OUT₂₀ is isolated from the V_(dd) and ground nodes in substage 20₁ by transistors 24₁ and 28₁ being off. Substages 20₂ and 20₃ will, in this example, similarly have address lines COL2 and COL3 at a low logic level, turning off their transistors 24 and 26 and allowing substage 20₀ to drive line OUT₂₀.

As is well known in the art, transient radiation events cause, among other effects, source-drain photoconduction through otherwise off MOS transistors. As described above relative to the circuit of FIG. 1, photoconduction to or from one logic state can upset the sensing, or writing, of the other logic state. Referring to the circuit of FIG. 2, the transient radiation would tend to cause the otherwise-off transistors 24 and 26 in the unselected substages 20₀ through 20₃ to conduct. In the event of such photoconduction in the circuit of FIG. 2, line OUT₂₀ would tend to pull to a voltage somewhere between V_(dd) and ground. Accordingly, the voltage differential between either the high or low logic state output by the selected one of substages 20₀ through 20₃ and the voltage to or from which photoconduction may conduct will be reduced from that of the circuit of FIG. 1, providing for an increased tolerance to transient radiation events.

Referring now to FIG. 3, the incorporation of stage 20 into a multiple-stage column decode architecture will be explained. The example illustrated in FIG. 3 is for a 128 column array, decoded into a single output bit from seven column address signals A0 through A6. Stages 20 each receive four of data lines D₀ through D₁₂₇ as described above relative to FIG. 2; accordingly, there are thirty-two stages 20 in the embodiment of FIG. 3. The four address lines COL0 through COL3 (i.e., lines COL_(n) of FIG. 3) are generated by one-of-four decoder 29, according to the value of the address signals on lines A0 and Al, and are communicated to each of the thirty-two stages 2.

A group of second stages 30 are constructed similarly as stage 20 shown in FIG. 2, but in this embodiment perform a one-of-eight selection; accordingly, there are four of second stages 30 in the embodiment of FIG. 3. Each of second stages 30 receive, as data inputs, the outputs of eight of stages 20. The corresponding address lines are presented to each of the four second stages 30 by lines 34_(n), one of which is asserted by one-of-eight decoder 32 responsive to the value of the address signal on address lines A2 through A4. Each of the four second stages 30 thus select and output the data state of one of its associated stages 20.

Third stage 40 performs the final selection of the bit from which data is to be read. Third stage 40 is also constructed similarly as stage 20 shown in FIG. 2, and receives the output from the four second stages 30 as its data input. Third stage 40 receives its corresponding address signals on lines 44_(n) from one-of-four decoder 42, generated according to the value on address lines A5 and A6. The selected one of data lines D₀ through D₁₂₇ is thus in communication with line OUT, through its stage 20, selected by the selected one of second stages 30.

The multiple stage selection accomplished by the circuit of FIG. 3 provides for the restoration of signals from the prior stage which have been degraded. Referring back to FIG. 2, the data state of a selected data line D0 through D3 is inverted by its corresponding one of NAND gates 22 to either a full high or low logic level. For second stages 30 or for third stage 40, the NAND gate receiving the output signal from the selected prior stage will similarly perform the logic inversion so long as the signal has not degraded to the point that its logic level is indeterminate (or incorrect). The driver consisting of transistors 24 and 28 within the selected substage 20_(n) will then fully invert the output of its NAND gate 22, so any degradation of the signal received from a prior stage is eliminated by second stages 30 and third stage 40 constructed according to the invention. Furthermore, the use of multiple stages (i.e., low fan-in and fan-out circuits) will similarly boost signals which are degraded due to high temperature operation, providing for more constant performance of the circuit over a given temperature range.

It should further be noted that the stage embodiment illustrated in FIG. 2 can serve as a single stage decoder, in devices where the number of data lines is small or in devices where transient radiation tolerance is not a concern. As described above relative to the example of FIG. 1, either the selected data line D₀ through D₃ itself, or a sense amplifier, will be driving the load capacitance of line OUT (and the parasitic capacitance of transistor 14), the drive capability of either often being small considering the load being driven. The design of FIG. 2 provides amplification of the voltage of the selected data line, speeding up the communication of the selected memory cell to the output terminal.

Although the invention has been described in detail herein with reference to its preferred embodiment, it is to be understood that this description is by way of example only, and is not to be construed in a limiting sense. It is to be further understood that numerous changes in the details of the embodiments of the invention, and additional embodiments of the invention, will be apparent to, and may be made by, persons of ordinary skill in the art having reference to this description. It is contemplated that such changes and additional embodiments are within the spirit and true scope of the invention as claimed below. 

I claim:
 1. A multiple-stage column decoder for selecting a data line of a memory responsive to a column address signal, comprising:a first address signal decoder for decoding a first set of bits of said column address signal; a second address signal decoder for decoding a second set of bits of said column address signal; a plurality of first select stages, each having a data input and a select input, said select input connected to the output of said first address signal decoder, each first select stage for selecting its data input for output therefrom responsive to the output of said first address signal decoder; and a second select stage, connected to the outputs of the plurality of first select stages and to the output of the second address signal decoder, for selecting one of the outputs of the plurality of first select stages responsive to the decoded address signals from said second address signal decoder; wherein said second select stage comprises:a plurality of gates, each having an input for receiving the output of an associated one of said first select stages and an input for receiving a decoded address signal from said second address signal decoder, and each having an output for presenting a signal corresponding to the output of its associated first select stage responsive to its decoded address signal indicating selection thereof; and a plurality of drivers, each associated with one of said plurality of gates, each having an input connected to the output of its associated gate, for driving its output to a logic state corresponding to the output of its associated gate, and each having an isolation switch, connected to its corresponding decoded address signal, for placing its output in a high impedance state responsive to said decoded address signal indicating non-selection of its associated first select stage; and wherein the outputs of each of the plurality of drivers in said second select stage are connected together to present the output of the selected one of said first select stages.
 2. The column decoder of claim 1, wherein said data input of said plurality of first select stages is connected to a data line of a memory array.
 3. The column decoder of claim 1, further comprising:a third address signal decoder for decoding a third set of bits of said column address signal; and a plurality of third select stages, each of said third select stages having a data input connected to a data line of a memory array, having a select input connected to the output of said third address signal decoder, and having an output connected to a data input of one of said first select stages, for communicating data from its data input to its output according to the output of said third set of bits of said column address signal.
 4. The column decoder of claim 1, wherein each of said first select stages comprise:a gate, having inputs corresponding to said data input and said select input, and having an output, for presenting at its output a signal corresponding to the state at its data input when its select input indicates selection of its data input; and a driver, having an input connected to the output of said gate and having an output, for driving its output to a logic state corresponding to the output of said gate, and having an isolation switch connected to said select input of said gate, for placing the output of the driver in a high impedance state responsive to said select input indicating non-selection of the data input of said gate; wherein the outputs of each of the drivers for each of the plurality of first select stages are connected together to present the output of the selected one of the first select stages to a corresponding one of said second select stages.-- 